1. Field of the Invention
The present invention relates to a nitride semiconductor structure. In particular, the present invention relates to: a nitride semiconductor structure including a substrate for allowing crystal growth and a high-quality nitride semiconductor grown on the substrate for allowing crystal growth, the nitride semiconductor having a different lattice constant or a different thermal expansion coefficient from that of the substrate; a method for producing such a nitride semiconductor structure; and a light emitting device produced by employing such a nitride semiconductor structure.
2. Description of the Related Art
Conventionally, nitride semiconductors have been employed as materials for constructing blue light emitting diodes (referred to as xe2x80x9cblue LEDsxe2x80x9d) or blue laser diodes (referred to as xe2x80x9cblue LDsxe2x80x9d). A nitride semiconductor is typically grown on a substrate by a metal-organic chemical vapor deposition (MOCVD) method, a hydride vapor phase epitaxy (HVPE) method, or a molecular beam epitaxy (MBE) method. In general, when a semiconductor is grown on a substrate, a substrate is used which is either of the same material as the semiconductor to be grown thereon or has a lattice constant and/or a thermal expansion coefficient similar to those of the semiconductor to be grown thereon.
It is impossible in the state of the art to prepare an appropriately-sized nitride semiconductor substrate which is of the same material as an overlying nitride semiconductor. Accordingly, a sapphire substrate, a SiC substrate, a spinel substrate, or the like is used as a substitute for a nitride semiconductor substrate. However, due to the large difference in lattice constant or thermal expansion coefficient between a nitride semiconductor and a sapphire substrate used as a substitute substrate, it is known that a nitride semiconductor film which has been grown directly on a sapphire substrate may contain threading dislocations at a density on the order of 109 to 1010 cmxe2x88x922. Therefore, it has been difficult by allowing crystal growth directly on a substitute substrate to obtain satisfactory nitride semiconductor crystals, i.e., nitride semiconductor crystals having substantially no crystal defects or a substantially zero threading dislocation density.
As used herein, a xe2x80x9cthreading dislocationxe2x80x9d is defined as a dislocation, particularly that occurring within a crystal or at an interface between crystals, that reaches the surface of the substrate.
Currently, a selective growth method is commonly adopted as a method for producing a nitride semiconductor film directly on a sapphire substrate because it is supposed to reduce the density of crystal defects or threading dislocations.
Hereinafter, a conventional method for producing a nitride semiconductor film will be described which utilizes selective growth of a nitride semiconductor.
In a first step, a first layer of a nitride semiconductor is formed directly on a sapphire substrate by using a MOCVD apparatus. In a second step, a SiO2 layer is vapor deposited directly on the first layer of nitride semiconductor by using a chemical vapor deposition (CVD) method. In a third step, SiO2 layer is processed so as to form a pattern having periodic openings by a known lithography technique. In a fourth step, the sapphire substrate which has undergone the third step is placed into a HVPE apparatus so as to grow a second layer of nitride semiconductor thereon. In accordance with this procedure, the density of threading dislocations (which would cause deterioration in the crystal quality) in the second layer of nitride semiconductor, which has been grown in the fourth step, is reduced to about 6xc3x97106 cmxe2x88x922. See Proceedings of 58th Applied Physics Association Lecture Meeting, 2p-Q-15 No. 1 (1997) p. 266xe2x80x3; or Jpn. J. Appl. Phys. Vol. 36 (1997) p. L899. The reduction in the threading dislocation density is due to the selective growth of the nitride semiconductor on the SiO2 masking pattern during the third step. Specifically, the second layer of nitride semiconductor which is grown directly on the masking pattern is more likely to develop in the openings of the masking pattern than in the portions where the SiO2 layer remains.
The initial growth of the second layer of nitride semiconductor begins mainly in the openings. As the growth reaches the uppermost level of the SiO2 layer, lateral growth begins so as to bury the SiO2 masking layer, while the growth also continues along the direction perpendicular to the substrate. This lateral growth does not emanate from the underlying masking layer but rather from the nitride semiconductor crystals grown in the openings, which serve as growth cores. Therefore, the lateral growth is less susceptible to lattice mismatching.
Although the threading dislocations that are generated within the first layer of nitride semiconductor may intrude the second layer of nitride semiconductor through the openings in the masking layer, they are diverted by the lateral growth so as to proceed along the lateral direction. Consequently, few threading dislocations reach the uppermost surface of the nitride semiconductor, resulting in crystals having a low threading dislocation density.
Alternatively, it is also possible to form a SiO2 masking pattern directly on a sapphire substrate and selectively grow a GaN monocrystalline film by MOCVD, as reported in Proceedings of 58th Applied Physics Association Lecture Meeting, 2p-Q-14 No. 1 (1997) p. 265. The technique described in this report omits the first step, so that a nitride semiconductor film is formed by only the second through fourth steps. This literature reports that the threading dislocation density directly above the SiO2 is reduced to about 105 to about 106 cmxe2x88x922, as compared to the about 109 to about 1010 cmxe2x88x922 threading dislocation density within the GaN monocrystalline film which is formed directly (i.e., in the openings of the SiO2 masking layer) on the sapphire substrate.
The above-described techniques for producing a nitride semiconductor film were expected to reduce the threading dislocations within the nitride semiconductor film and to improve the emission characteristics and quality of a nitride semiconductor light emitting device formed directly on the nitride semiconductor film.
However, although the above-described nitride semiconductor film-producing techniques may reduce the threading dislocations within the resultant nitride semiconductor film, they employ at least three steps for forming a nitride semiconductor film having such a reduced threading dislocation density. In addition, it is necessary to change apparatuses from the first step to the second step, or from the second step to the fourth step.
In particular, the first conventional technique, which involves the first through fourth steps as described above, requires two steps of crystal growth. In general, any regrowth step which is performed after suspension of a previous growth is accompanied by the problem of impurity deposition on the crystal surface. This impurity concern is particularly great for the first conventional technique because the SiO2 layer deposited in the second step is patterned. Moreover, the GaN layer which is utilized as a thick second layer of nitride semiconductor is grown at a growth temperature of about 1000xc2x0 C., thereby leaving the SiO2 masking pattern which is formed in the third step quite susceptible to thermal damage. The inventors have discovered through experimentation that Si or O2 present in a thermally damaged masking pattern may unfavorably affect the resultant nitride semiconductor film.
When a nitride semiconductor light emitting device is produced directly on a nitride semiconductor film which has been formed by any conventional nitride semiconductor film-producing technique, impurities which have been formed as a result of the thermal damage to the masking pattern may influence an active layer of the nitride semiconductor light emitting device structure for generating light. Such influence may result in a decrease in the light emission efficiency of the individual light emitting devices, low product reliability due to light emission efficiency variation with respect to a number of light emitting devices, and/or a low production yield of nitride semiconductor light emitting devices.
A nitride semiconductor structure includes: a substrate having a growth surface, a convex portion and a concave portion being formed on the growth surface; and a nitride semiconductor film grown on the growth surface, wherein a cavity is formed between the nitride semiconductor film and the substrate in the concave portion.
In one embodiment of the invention, the convex portion and the concave portion are defined by a plurality of parallel grooves.
In another embodiment of the invention, the plurality of grooves have a width b and a depth h such that bxe2x89xa6about 10 xcexcm and hxe2x89xa70.2xc3x97b, and adjoining one of the plurality of grooves are spaced apart from each other so as to retain a distance of about 20 xcexcm or less between center lines thereof.
In still another embodiment of the invention, the plurality of grooves have a width b and a depth h such that bxe2x89xa6about 10 xcexcm and hxe2x89xa7b, and adjoining one of the plurality of grooves are spaced apart from each other so as to retain a distance of about 20 xcexcm or less between center lines thereof.
In still another embodiment of the invention, the plurality of grooves are formed along a  less than 1-100 greater than  direction of the nitride semiconductor.
In still another embodiment of the invention, the grooves are formed so as to extend along a cleavage plane or an etching stable plane of the substrate.
In still another embodiment of the invention, the growth surface of the substrate is composed essentially of a nitride semiconductor, the plurality of grooves being formed along a  less than 11-20 greater than  direction of the nitride semiconductor.
In another aspect of the invention, there is provided a method for producing a nitride semiconductor structure including the steps of: forming a convex and a concave portion or a plurality of grooves on a growth surface of a substrate; and thereafter growing a nitride semiconductor film on the growth surface of the substrate so that a cavity is formed in the concave portion.
In yet another aspect of the invention, there is provided a light emitting device including: any one of the above nitride semiconductor structures; and a light emitting structure formed on the nitride semiconductor structure, the light emitting structure including an active layer including a nitride semiconductor.
Thus, the invention described herein makes possible the advantages of (1) providing a nitride semiconductor structure including a substrate and a high-quality nitride semiconductor film epitaxially grown thereon, the substrate having a different lattice constant or a different thermal expansion coefficient from that of the nitride semiconductor; (2) providing a method for producing such a nitride semiconductor structure; and (3) providing a light emitting device incorporating such a nitride semiconductor structure.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.